German patent 550 692 describes a method and apparatus in which pit-wet coal is metered into a stream of hot combustion gases. The combustion gas stream which is laden with wet coal particles is fed to a high-speed fan in which the coal particles are comminuted and at the same time dried. After passing through the fan, the combustion gas stream is passed to a separator, for example a cyclone, in order to remove the comminuted and dried coal particles from the combustion gas stream in known manner.
EP 0 159 710 A2 discloses a method and apparatus for grinding, among other things, wet coal or wet coke. The grinding method proposed in this publication provides two grinding surfaces between which grinding elements, in particular in the form of grinding rollers, are arranged. The two grinding surfaces may be planar or indeed curved, for example cylindrical, and may be arranged horizontally, vertically or inclined at any angle.
German Utility Model specification DE 94 17 092 U1 discloses and describes a means of feeding material for a material bed roller mill which is operated in a closed grinding and classifying cycle. Based on this mode of operation, the material being ground which is fed to the material bed roller mill includes a high proportion of fine particles which may readily be fluidised, which presents a problem for the construction of a material bed as required for proper functioning. To overcome this problem, it is proposed to use as the silo discharge means a horizontal-axis twin screw which is operated in opposing directions, and to supply this twin screw, in opposition to the pressure of an overflow valve, with an excess of material by way of at least one vertical-axis compressor screw. It is stated that the counter-pressure exerted by the overflow valve has the effect of pre-compressing the conveyed material and hence completely filling the screw flights of the twin screw with a material of high bulk density. It is stated that completely filling of the screw flights of the compressor screw is ensured by the conveying output of the twin screw, which is always greater than that of the compressor screw.
German Utility Model specification DE 72 37 934 U1 discloses a silo for powdered or granular material which is provided with a discharge means in the form of a conveyor screw which on the one hand rotates about itself and on the other at the same time performs a translational movement above the base of the silo. Disadvantages of a construction of this kind which should be mentioned are on the one hand the tendency of the material in the silo to form bridges and on the other the poor accessibility of the discharge means in the event of damage to the discharge conveyor and its drive system. To overcome these problems, it is proposed to pass a hollow support which opens into two lateral silo openings through the lower end region of the silo, this hollow support being dimensioned and arranged such that it covers the drive mechanism of the discharge conveyor by means of its central region and keeps this drive mechanism accessible and prevents the formation of a naturally occurring bank of material in the direction of the centrally arranged outlet opening.
German Utility Model specification DE 94 10 287 U1 discloses a screw compressor having an infeed aid. The screw compressor is intended to serve to reduce the volume of bulky substances, such as used cardboard packaging, by compressing them. To ensure that the material put into the screw compressor is fed without problems, a shaft having at least two infeed elements parallel to the screw axis is arranged in the input hopper above the compressor screw and is driven in a direction of rotation opposed to that of the screw shaft. In this way, an infeed action is to be established between the compressor screw and the shaft.
In known methods for comminuting wet coal or wet coke, for example using a vertical roller mill, it is disadvantageous in practice that the complexity of the apparatus for drying the wet coal or wet coke and for the prevention of dust explosions must be high. In addition, at relatively high pressing forces, which are desirable per se for proper comminution, a vertical roller mill tends to flake formation, that is to say that flake-like agglomerations of fine particles are formed, and these make it extremely difficult or even impossible to remove the fine material in the classifying zone of the vertical roller mill, because the fine fraction of the comminuted material that has agglomerated in this way can no longer be removed by the air flow acting in the classifying zone. To prevent flakes from forming, a low specific grinding force is therefore established in a vertical roller mill, and this has the effect of reducing the level of comminution. In addition, the formation of flakes impairs the function of a mill with concentric cylindrical grinding surfaces.